The present invention relates to an optical system for an optical pickup apparatus, optical pickup apparatus and optical information recording reproducing apparatus, and particularly to an optical system for the optical pickup apparatus, optical pickup apparatus and optical information recording reproducing apparatus by which the high density optical information recording or reproducing can be attained.
Conventionally, an optical disk represented by a CD (compact disk) or DVD (digital versatile disk) is widely used for the accumulation of the music information or image information or the storage of the digital data such as the program data. Furthermore, as the arrival of the high degree information society, an amount of the managed information is becoming great, and the increase of the capacity of these optical disks is strongly required.
Herein, the increase of the recording capacity (recording density) per unit area in the optical disk can be realized by the engineering by which the spot diameter of the converged light spot obtained from the optical system for the optical pickup apparatus is decreased. As being well known, because this spot diameter is proportional to λ/NA (where λ is the wavelength of the light source, and NA is numerical aperture of an objective lens), in order to reduce the spot diameter, the reduction of the wavelength of the light source used for the optical pickup apparatus and the increase of the numerical aperture of the objective lens positioned opposite to the optical disk in the optical system for the optical pickup apparatus are effective.
In them, relating to the reduction of the wavelength of the light source, because the research and development of the blue violet semiconductor laser or SHG blue violet laser which generates the light of about 400 nm are rapidly advanced, the practical use of them will be soon. When such a short wavelength light source is used, even when the objective lens of the numerical aperture NA 0.65 which is the same as the conventional DVD of the storage capacity of about 4.7 GB is used, and the optical disk of the diameter 12 cm which is the same as the DVD is used, the recording of the information of about 15 GB can be conducted in such an optical disk.
Further, relating to an increase of the numerical aperture of the objective lens, the research of the objective lens of numerical aperture 0.85 composed of one or two lens group is advanced. When the above-described short wavelength light source and the objective lens of numerical aperture 0.85 are combinedly used, the recording of the information of about 20 to 30 GB is possible onto the optical disk of the diameter 12 cm, and a further increase of density can be attained.
However, when the wavelength of the light source is decreased, and the numerical aperture of the objective lens is increased, there is a problem that the spherical aberration is easily increased due to various error factors and the optical performance is deteriorated. For example, the spherical aberration is directly increased due to the error factors such as the production error of the thickness of the protection layer of the optical dissk, production error of the thickness of the objective lens, or the change of the refractive index due to the temperature change, and also due to the jump between layers of 2 layer disks. Accordingly, as a composition to correct such a spherical aberration, an expander lens composed of 2 lens groups is arranged in the optical path between the light source and the objective lens, and an optical pickup apparatus in which the interval of the lens groups constituting the expander lens is changeably adjusted, or an optical pickup apparatus in which a position of a coupling lens which changes the divergent angle of the diverging light flux emitted from the light source and guides to the objective lens, is changeably adjusted in the optical axis direction, is written in the following patent references.
[Patent Reference 1]
Tokkai 2000-No. 131603
[Patent Reference 2]
Tokkai 2001-No. 324673
However, in these optical pickup apparatus, there is a problem that the coma of the converged light spot is increased due to the relative position deviation of the lens whose position is changeably adjusted by an actuator (hereinafter, the composition lens group of the expander lens whose position is changeably adjusted in the optical axis direction by the actuator, or a coupling lens is called “moveable lens”), and the other lens. Such a problem has an inclination that it is actualized particularly in the case where the size of the optical pickup optical system is reduced when the lens group interval of the expander lens is reduced, the magnification of the expander lens is increased, or the numerical aperture of the coupling lens is increased.
Further, for a personal computer use, or in order to mount in the optical information recording reproducing apparatus for car use, also in the optical pickup apparatus which uses the blue violet light source having a short wavelength and an objective lens having a large numerical aperture, it is presumed that its size reduction is required in near future, however, as described above, there is a problem that, when the size of the optical system for the optical pickup apparatus is reduced, because the light converging performance is greatly deteriorated to the decentering error (deviation) of the movable lens, the allowable range of the decentering error is narrowed, thereby, the production cost of the optical system for the optical pickup apparatus is increased.